As part of a process of manufacturing integrated circuits (ICs), inspection systems can be applied to search for mask defects. An example inspection system process is shown in FIG. 5A.
The process of FIG. 5A begins with a photolithography mask (500) to be searched for defects. An inspection system is applied to the mask to obtain an inspection image (step 501). The inspection image is input to a “mask recovery” step (step 502) that produces an estimate of the mask. This mask estimate is then input to a photolithography simulator (step 503, that can also be referred to as a “stepper simulator”) that produces a simulated wafer image (504).
A computer program can compare this simulated wafer image with a reference wafer image, searching for differences representing defects that could result during the mask manufacturing process. Each such defect found can be given a score representing its level of significance.
The inspection system can be an optical system that has a limited frequency response. For example, an inspection system can cause the spectral characteristic, of the received inspection image, to suffer greater attenuation as frequency increases. This attenuation at higher frequencies tends to cause a loss of resolution in the inspection image, increasing the size of the smallest defects that can be detected.
It would therefore be desirable to have a filter that could be applied to the inspection image, before such image is input to a photolithography simulator, to reduce frequency distortion introduced by the inspection system.